Automatic control system using nonlinaer responsive elements



2 SHEETSSHEET 1 IN VEN TOR.

ATTORNEYS E. T. DAVIS AUTOMATIC CONTROL 'SYSTEM USING NONLINEAR RESPONSIVE ELEMENTS 33m 325232 6 E2w w 05.5302

ELWOOD T. DAVIS Oct. 7, 1952 Original Filed 001.. 10, 1944 2 SHEETS-SHEET 2 E. T. DAVIS AUTOMATIC CONTROL SYSTEM USING NONLINEAR RESPONSIVE ELEMENTS 10, 1944 0rigina l\ Filed Oct.

oOOnN 5 m 26 00- OD 0m 0' ON 000m 000m- 0 o E !w A u m. m %m S d M. l. 09 W IN VEN TOR. ELWOOD T. DAVIS ATTORNEYS ReiuuedOchZ 1952 srs'ranieusm'c' s mmsa n smssw stsmm 'offl ennsylvania' I o orimse zssmts; a e Nov mber '21, 1950.

v 'Serial No. 658,098,0ctober10, 1944: Applica- I tion for reissu e lune 13. 1952} Serial 0,2 5

. w (QU H' Matter 'encloeedin heavy'braeketl-l I appears-in the original patentbut forms no part of this reissne'lpeeiilcation: in'italics'i lndicates the session; made by reissue This invention relates" to. temperature control- 7 systems, more'perticularlyto systems controlled 5 1 by non-linear temperature responsive devices:

and heater an'object the provision of a simple and reliable system which responds to lineartemperature changes notwithstanding the temperat ture-responsive devices are non-linear.

thereoLxreference should behad to the accom panying drawings wherein:

Fig. 1 diagrammaticallyillustrates a system I embodying the invention:

Open-hearth furnaces include a plurality of'regenerative chambers in which are'disposed flre- 1 brick. Certain of the chambers serve for-a time I toabsorb heat from the. products of combustion;

while other chambers and their"fire-brick"'concurrently serve topreheat the combustion mate- I rials. The heating and heat-absorbing functions of the regenerative'chambersare periodicaily reversed when a predetermined temperaturefdiil'ero I Fig.'l diagrammaticallyillustrates certain basic ence exists therebetween.

Thermocouples located 111' selected pastas. 3;: each checker-brick chamberhave been .used y measure the temperatureo'f the chambers and to control the aforesaid operations. Thermocouples are, to a substantial degree. 'lin'ear' -temperature responsive devices, and with suitable-compensate ing circuits the measuring and control ='circuits' may be relatively simple. However, --since the temperature of the gases. as measured byathermocouple is only an indirect measure of the tem-- Fig.- 2 diagrammatically illustrates in perspec-. tive certain partsof .amechanical relayor measv ;uring device utilized in the primary system; .10

1 elements of a modification of one feature of the r Fig; 3 isan elementarydia'gram of the essential system of Fi 11' Figs. 4,; 5 and 6' are graphs explanatory of the I manner in which the systems of Figs. 3 and 6 compenseLtefor the non-linearity of radiation elements'ofFig. 1 with the features of Fig. 3 in- 'corporated therein. Referri'ngto the drawings, the invention in one form is shown as applied to a regenerative open hearth furnace 10' provided with checker-brick chambers ll and 12 on one side thereoLand perature of the.checker-brich-itfis recognized that direct-measurement of the temperatureof .8

the checker brlcl; itself-is more "desirable. 7 Ii. the

temperature rises beyond the melting or soften-. ing temperature of: the brick, the resultant changes due' to deformation, slaggin'g,;tor=. actual @h g flow of the brick material will the life of the checker-brick. 3 v p In carrying outthe present invention in one form thereof, a' measuring system is fprovide'd which compensates for the non-linearity ofthe' output of radiation pyrometers. There is pro duced by the system a definite-control, 'for' a checker-brick chambers l3 and I4 on the other side thereof. Fuel is supplied to the furnace through an inlet .pipe Ill which flows by way of valve is and pipe I! to the chamber ll. At the same time, combustion air is introduced through thein'let pipe IB-and flows by way of valve is and the; pipe, 20 to the chamber II. The fuel and fair'are'heated inthe chambers I3 and I4 before combustion thereof in the zone 2 I. The preheating of both the combustion air and fuel is advantageous, as is well understood by those skilled in the art. The'products of combustion flow across theope'n'hearth'of the furnace and exit by way oi the checker-brickch'ambers II and i2. During the passage of the products of combustion given temperature difference-regardless of- I where that difference occurs 'zove'r' a relatively",

wide range of temperatures. More particu-- larly, a primary measuring system isprovided which measures the output of flrst o'ne radiatingpyrometer, and then the omerr m v eter- This Primary s t is arrangedto llnb'ale ance a secondary system'by an amount which is and I! may be safely heated to a predeterminedthrough chambers H and II, the

therein is elevated in temperature.

Y It will be observed these combustion gases are conducted by pipes and 23, and through valves l8 and I9, to a common pipe or duct 24 which leads to a'stack 21. Thefbrickwork, or other heat,

absorbing material, disposed in the chambers ll temperature; Fire-brick, for example, will fuse a linear function of the temperatures which activeto the respective pyrometers. Whenever the secondary network is unbalanced in predetermined sense and to a predetermined degree corresponding with a predetermined temperature diil'erence, a reversalin the operation of the regenerative checker-brick chambers is produced;

and slag ifheated near or above the melting temperature thereof. In practice, it is desirable before such melting temperature is reached to reverse the operation of the furnace by throwing the valves l8 and 19 to their reverse positions. Any suitable mechanism may be provided for this purpose, such as a motor, or solenoids may be utilizedas indicated at 25 and 26. With the Foramore complete'understanding of the ini vention and'iforfurther-objects and advantages checker-brick valves in their reverse positions. the fuel and air flow to the open hearth furnace by way of the pipes 22 and 2! and the regenerative chambers l2 and i I, respectively. The combustion products exit by way of the chambers II and I4 and the pipes l1 and 20. It is obvious that the foregoing cycle of operations is repeated in order to maintain a relatively high efilciency of operation for the furnace which means a substantial saving in fuel consumed. i

In accordance with the present invention,-

radiation pyrometers II and 29 are mounted so as to view the checker work within the interior of chambers II and 14, respectively. By so mounting, or sighting, the radiation pyrometers 28 and 29 to receive the radiation from the associated chamber. particularly to view a typical portion thereof, the amount of heat or. the heathead within the checker-brick chambers may be determined with a great deal more accuracy than with thermocouples. These radiation pyrometers are connected through a switch 30 to a primary measuring system and mechanical relay 3 l. The operation is such that the primary measuring system is first responsive to one and then to the other of the radiation pyrometers. As shown. the switch 30, in its left-hand position, connects the pyrometer 29 to the primary measuring system. After a predetermined period of time the cam 82 operates the switch 30 to its right-hand position to connect the pyrometer 28 to the measuring system II. This cycle is repeated at a rate governed by the speed of a timing motor 42 of relay 3|, shown in Fig. 2.

This primary measuring system may be of any suitable type in which the output or potential difference produced by each of the radiation pyrometers 28 and 29 is in turn balanced by a potential difference whose magnitude may be adjusted by means of a slidewire 38. The slidewire 36 is automatically positioned by a suitable mechanism or relay system of the type shown in Squibb Patent No. 1,935,732. The primary measuring system is preferably of the potentiometer type, as shown in Fig. 15 of said patent. It is characterized by a current of constant value being maintained in slidewire 36.

Referring to Fig. 2, a fractional part of the relay mechanism, as shown in said Squibb Patent No. 1,935,732, has been illustrated. The relay mechanism is provided with a shaft 34 which carries a disc 35 on which the aforesaid slidewire 36 of the primary measuring system is mounted. The shaft 34 also carries a driving pulley II for a violin string 38 which serves to drive a recording pen I9 across a strip chart 40. Though not illustrated in Fig. 2, a pointer is attached to the pen is or to the violin string 88, and is driven across a'suitable indicating scale 4|, as shown in Fig. 1. It may be observed the position of the pen 39, with respect to the calibrated chart 40. serves both as a recording and 'an indicating means.

The recorder chart 40 is normally driven at constant speed by any suitable source, such for example as a motor 42. which also serves to operate the recorder mechanism. This motor 42 drives, through the associated gearing, a shaft 43 on which is mounted a pair of cams 44 and 45 which cooperate with a clutch member 46. pivoted at 46a, coaxially with, but independently of, shaft 34. The clutch member 46 is anguiarly positioned in one direction or the other by means of a galvanometer pointer 41, and a feeler and clamping mechanism. For example, when an 4 unbalance exists in the primary measuring system, the galvanometer-ll deflects its pointer 41 to the right or to the left, depending upon the direction of the unbalance; that is, whether the potential difference across the radiation pyrometeris greater or less than the potential difference controlled by the slidewire 36. Upon deflection of the pointer 41, a clamping bar 49 is moved upwardlyby means of a cam (not shown) to press and to clamp the pointer 41 against a cooperating stop or clamping member 50. While the pointer is so-held, a pair of feelers, 5| and Ii. are released by a cam (not shown) rotating with the shaft 43 for movement toward one another by a biasing spring 53. The upper end of one or the other of the feelers is stopped by engagement with the end of the pointer 41. The other feeler continues its movement and the lower end thereof engages a pin 54 extending from the clutch arm 48, thereby to rotate the arm an amount depending upon the extent of the deflection of the pointer 41. The pointer 41 is then unclamped and responds to any further difference of potential which may exist as between a radiation pyrometer and the primary measuring network.

Subsequently in the cycle of the mechanism, the clutch member 46 is moved inwardly against a clutchdisc 55. While the clutch members are so engaged. one or the other of the earns 44 and v 45 engages the driving clutch arm 43 and restores it to its neutral position, and in doing so, rotates the disc 55 and the shaft 34 in one direction or the other. This, of course, produces rotation of the slidewire I6 and the driving pulley 31. The movement of the slidewire 36, relative to its cooperating contact 51. is in the direction to restore balance to the primary measuring network.

In this manner, the slidewire 36 is moved until balance is restored, with a corresponding movement of the driving pulley 31 to drive the pen 39 to a new position indicative of the change in potential difference produced by first one and then the other of the radiation pyrometers 28 and 29.

It will be observed the scale 4| is non-linear. It is a power scale which corresponds with the non-linear variation in the output of each radiation pyrometer. Because of the non-linearity, it is impossible to provide a system which directly responds to a desired temperature difference. In other words, the movement of the shaft 34 would greatly differ for the same temperature difference in the differing ranges of 1500 F.-2000 F., 2000 F.-2500 F., and 2500 F.- 3000 F.

In accordance with the present invention, a second slidewire 60 is mounted on the shaft 34 and is adjustable thereby with reference to its cooperating contact GI. In Fig. l, the slidewire 3G is diagramatically illustrated, together with the scale 4|, and the slidewire so is similarly illustrated with the broken line 34, indicating the mechanical connection therebetween. The slidewire Gil. itself a wound or distributed resistor mounted on a supporting disc, has connected in parallel therewith a plurality of resistor sections indicated at 'a, b, c, d and e. The resistors'are connected in parallel with different sections of the slidewire resistor ill and the relative values are so selected as to produce potential differences substantially directly proportional to the tem- A pending upon the sense of the 8| mounted Qatar is moved to a new position which unbalances a secondary measuring network,'which comprises the slidewire 80 and a second slidewire resistor 85 having a cooperating contact 68. These two slidewires, as shown, in eflect form a Wheatstone bridge to which alternating current is supplied from a suitablesource indicated at 81.

Neglecting for the moment the additional circuits included in the circuit across the opposite juncture points of the bridge, the unbalance produced in the bridge is applied by conductors, 68 and III to a Sensitive Detector. This detector 12 may be a'vacuum tube-relay combination which functions to close contacts 18 or I4, de-

unbalance.

When the detector 12 responds to an unbalance in one direction, it serves to close contacts 18 and when the unbalance is in the opposite direction, it serves to close contacts 14. These contacts 18 and I4 serve to control the operation of a motor which adjusts the slidewire 85 relative to its contact 68 to restore balance in the secondary measuring circuit. The energizatlon oi the motor 15 is also under the control of cam-operated contacts 16 and 11. Additional cam-operated contacts 18 and I9 cooperate with contacts l8 and H to control the energization of the solenoids 26 and 25, respectively.

Now that the structural elements of the system have been explained, the'manner in which they cooperate together will best be explained by consideration of the operation as a whole. It'

will be assumed the regenerative furnace It has been in operation and that the valves I6 and I! have just been operated to their illustrated positions so that the air and fuel passing through the chambers l3 and I4 are absorbing heat from the checker-brick. Accordingly, the product of combustion passing through the chambers II and I2 increase the temperature of the checkerebrick therein. The primary measuring system. 3| is, by switch 30, connected to the radiation pyrometer 29. After a few cycles of operation, of the mechanism of Fig. 2, the slidewires 86 and 60 are positioned in accordance with the temperature of the checker-brick in the chamber It. The primary measuring system operates on a time cycle; that is, a measurement is completed in a predetermined period of time. Hence, the switches Hi-19 may be operated by cam on a shaft 83, Fig. 2, which, by suitable gearing B4, is rotated at relatively slow speed.

After the measuring system, connected to pyrometer 29, has been balanced, the cam 8| closes the contacts 16 and 11. The detector 12, during the foregoing operations, has responded to the unbalance in the secondary measuring circuit. Since the temperature in the chamber I4 is presumably higher (because of the assumption of an earlier transfer from heat-absorbing to heating), the detector 12 closes the contacts 14 to complete an energizing circuit for the winding 85 of the motor 15 from a suitable source of alternating current indicated by the input terminals 86a and 86b. The motor 15 then rotates the slidewire 65 relative to its contact 86 in a direction to produce balance in the secondary network. After a time interval sufficient for balance to be obtained, the cam 8| opens the contacts 16 and 11. At this time another cam 82 operates the switch 38 to disconnect the pyrometer 29 from, and to connect the pyrometer 28 to, the primary measuring network.

After the mechanical relay 3| produces balally adjusted relative to ance in the primary net o k. with p r meter 28 connected thereto, the cam 88 operates to close the switch 88 to connect the alternating current source 88 in series with a slidewire 8i and the detector 12. The slidewire 8| is manuits cooperating contact 82 so as to introduce a potential difference in the secondary measuring network of a magnitude which will be equal to that produced by the movement of slidewire 88 due to the diflerence in the temperatures, which is not to be exceeded. between the chambers l I and II. For example, it will be assumed this temperature difference is 700 F. The potential introduced by slidewire 8| compensates for this temperature difference. Accordingly, the detector I2 will respond onlyif the temperature observed by pyrometer 28 diifers by other than 700 F. from that of pyrometer 29. Depending upon the sense of the departure, whether above or below 700 F., the detector I2 will operate to close contacts 13 or 14. Shortly after closure of the switch 80, the cam 8| operates to close the contacts 18 and 18,- preparatory to closure of contacts 18 or II by detector 12.

Since it has been assumed that a transfer of operations has Just been made, it will be understood the temperatures of the chambers I3 and M are materially higher than the temperatures of the chambers II and I2. Nevertheless, the voltageintroduced from the source 86a, 86b and by the slidewire 9| is in the direction, and has the proper polarity to modify the operation of the Wheatstone bridge so as to require within the chamber II a temperature 700 F. higher than that in the chamber ll. Consequently, upon the initial reversal of operations the temperature of chamber II is no more than 700 higher than that of the chamber ll. Hence, the detector I2 responds to this unbalance to close its contacts 18. This completes an energizing circuit through the contacts 18 for the coil 26 of the valve-operating mechanism. Nothing happens, however, because the valves have already been operated by the coil or solenoid 26 to their illustrated positions. The cam Bl then opens contacts l8 and 79. The cam 80 opens switch 88 and the cams BI and 82 serve as before to operate switch 38 to its illustrated position and to close contacts 18 and 11.

The foregoing operations are repeated while the temperatures within the chambers I3 and I4 decrease and the temperatures of chambers II and I2 increase. No diiferent operation occurs until the temperature of chamber I l exceeds by a predetermined temperature, for example 700 F., the temperature of the chamber It. When this occurs the corrective voltage introduced into the Wheatstone bridge network is insuiiicient or is overcome by the unbalance introduced by the slidewire 60. Hence, the detector 12 operates to close the contacts 14. Consequently, the energizing circuit may be completed from the source 86a, 86b through contacts 14 and 78 for the solenoid 25, which thereupon functions to operate the valves It and I8 to their opposite positions. I

The regenerative furnace then operates with introduction of air and fuel through the chambers II and I2, with the combustion gases leaving the furnace by way of chambers I 3 and I 4. At the same time, the potential difierence introduced into the secondary network is reversed in polarity by means of a double-pole double-throw switch 95, mechanically operated, as indicated by the broken line c, by the valve-operating solenoids 25 and 2B. The reversal of the polarity '7 the temperature diiference to be in the Opposite directiomthat is, contacts [ll] 73 will not closeuntil the temperature of the chamber l4 exceeds by apredetermined amount, that is, 700' F., the temperature of the chamber II.

The foregoing cycle is repeated accurately to control the operation of the regenerative. furnace ll and to increase its overall operating efllciency.

By changing the values of the resistors a to e inclusive (Fig. 1) the resistance of the slidewire SI may be tapered or otherwise varied. Its resistance value may be varied in manner more than to oflset the non-linearity of the radiation pyrometers/ If this is done, the temperature diiference required for reversal of operation of the regenerative furnace II is increased as the temperature rises. In some applications this fea ture is advantageous since a constant temperature diiference control may not produce the desired rapidity of reversals at a selected low temperature range and reversals may occur with too great a frequency-at a higher selected range of operating temperatures. By suitable adjustment of the characteristics of the slidewire Bl. the frequency of reversals may be made more or less constant over a relatively wide range of selected operating temperatures. These resistance values mayalso be selected to produce under-compensation as well as over-compensation or exact compensation.

While the invention is applicable to radiation pyrometers of any character, those available on the market from applicant's employer under the trade name of Rayotube" have been found quite satisfactory.

By manually adjusting the slidewire 8|, any

requires desired temperature diiference may be required to produce reversal in the operation of the regenerative furnace ll. For convenient manual selection of the desired operation, the slidewire 5| may be calibrated in terms of temperature Associated with the reversing solenoids 25 and 2B are signal lamps l8 and 99 which serve to indicate when and which transfer-operation is taking place. Obviously, the actual operation of the valves I6 and I! may be efiected manually in response to the signal arising by energization of one or the other of such signal lamps. The valves may then be operated directly by an operator, or by a mechanism manually controlled by him.

While the invention has been described in connection with a regenerative furnace, it is to be understood the invention may be applied to all measuring and control systems where a linear or near-linear response is desired from non-linear condition-responsive devices.

Instead of the characteristic-changing resistors H associated with the slidewire ill, the same results may be obtained by an arrangement which is'itself believed to be new and which may be readily applied to many systems, including the one above described. The circuit arrangement is shown in its elementary form in Fig. 3. The winding I", the secondary of a transformer, represents a source of constant voltage E which is applied across a resistor llll and a slidewire I". The resistor Illl has a manually adjustable contact II! for short-circuiting a portion of resistor Ill. The slidewire i3 is adjusted relative to its 7 contact II by the primary mechanical relay II.

The relative adjustment of this slidewire also I short-circuits, or removes, from the series-circuit including resistor III, a part of the slidewire. The

' nitude determined by the following relations:

where V and E are in volts. and Bill and Bill represent the resistances in ohms of resistors Ill and ltl. Preferably, resistor III is arranged so that some of its resistance will always be eflective in the series-circuit.

When slidewire I" is excluded from the circult. BI" is equal to zero. Hence, for any value of Rllll the voltage V will .be zero. If resistor III could be excluded from the circuit, Rill would be zero and V would then equal E for any value of Rllll other than zero. Between these limits, V will vary non-linearly. depending upon the relative setting or position of the slidewire Hi3 with respect to its contact llll. Since the adjustment of slidewire I03 varies the resistance of the circuit including resistor IQI, both the resistance and the current flowing through that circuit change. Hence, the change in the voltage V varies as a function of the resultant change in the current and of the resistance. The variation in value of voltage V is not linear. It may be made to vary as a close approximation of the fourth root and therefore may compensate or correct for the non-linear approximate fourth power variation in the response of the radiation pyrometers 28 and 29.

In one embodiment of the invention the slidewire llll had a resistance of 172 ohms while the resistor Hll had a setting for a resistance of the remaining part of the series-circuit or ohms.

As already explained, the setting or position of slidewire 3i varies non-linearly with temperature. This is graphically shown in Fig. 4 where the position of the slidewire 18 in per cent of its range of movement has been plotted as ordinates, against temperature as abscissae. Since slidewires 36 and I03 are both simultaneously adjusted by relay 3|, the positions of this slidewire I83 in per cent of its range have been plotted in Fig. 5 as abscissae against the voltage introduced into the secondary measuring network for positions of the slidewire I03 as set by the relay 3|, as ordinates. The resultant curve of Fig. 5 is generally the reverse of that of Fig. 4. The curve of Fig. 4 varies approximately as the fourth power of the temperature while the curve of Fig. 5 shows the slidewire voltage varies approximately as the fourth root of the slidewire position.

In Fig. 6, the voltage introduced into the secondary network has been plotted as, ordinates against the temperature to which a pyrometer is subjected, as abscissae. The resultant curve throughout the range of temperatures, 1500 F. to 2500 F., is a straight line. thus graphically illustrating the manner in which a non-linear variation in the response of a radiation pyrometer, or other device, may be converted into a voltage which varies linearly with the measured temperature.

As shown in Fig. 3, the manual adjustment of resistance llll permits ready adjustment or the characteristics of the network of Fig. 3. By increasing or decreasing the value of resistance Illl. either an overcorrection or an undercorrection for the non-linear response of.the pyrometers may be obtained. Such abnormal corrections are in some applications advantageous. Without the present invention, specially designed and constructed slidewires would be necessary for every change in the characteristics. whereas in accord- .amwe, h s

9' I ance with the invention, great flexibility in oper.- ation is at all times made available.

The principal features of a commercial embodiment of theinvention have been illustrated in Fig. 7. Parts. corresponding with those of Figs. 1-3 bear the same reference characters.

In Fig. 7, the slidewire B of Fig. 1 has been replaced by an arrangement the equivalent of Fig. 3. Thus it will be seen the mechanical relay 3| serves relatively to adjust slidewire I and its contact I. This slldewire has a resistance of 172 ohms with a fixed resistor lit of 50 ohms connected in series therewith, and in series with a manually adiustable slidewire Ill having a range of 140 ohms. For a straight-line variation with temperature, this slidewire Ill will be set for a value of 105 ohms. With this setting. the resistance of the series-circuit, exclusive of slidewire I03, is 155 ohms, the same as for Fig. 3. This series-resistor combination is connected across the secondary winding III! of a transformer Ill. The balancing slidewires 85a and 85b are adjusted by the motor I 5 to adJust them relative to their contacts a and "b, as already explained in connection with the system of Fig. 1. The division of slidewire 85 into the two slidewires 65a and 65b is a matter of convenience of design. It will be understood the two slidewires jointly decrease or increasethe resistance in the network under the control of motor 1-5, as already described in connection with Fig. 1.

The source of supply 61 in Fig. '7 is the secondary winding of transformer ill whose primary winding is connected across alternating current supply lines 5 and H8. This source of supply also is connected to the primary winding of transformer I II and to the primary winding of transformer Il.l whose secondary winding 86 serves to introduce the temperature-difference determining voltage into the network, In Fig. 7, the switch 95 is shown with contacts 95a and 95b closed, corresponding with the first operation assumed in the description of Fig. 1. When switch 90 is closed, this compensating voltage prevents reversal of the furnace until the temperature difference, in sense and magnitude, between chambers Ii and It has a predetermined value.

This temperature difference is predetermined by the setting of slidewire 9| with respect to its contact 92, as already explained. However, there is included in series with slidewire 9| of 164 ohms, a fixed resistor H5, of 75 ohms, and a manually adjustable slidewire Hit, of 140 ohms. The slidewire H6 is preferably adjusted relative to its contact H1, concurrently with adjustment of characteristic-determining slidewire IIH. Hence, when the slidewire Ifll is adjusted relative to its contact I02 to overcorrect or undercorrect for non-linearity, the slidewlre H6 is adjusted to maintain operation with the same temperature difference as predetermined by the setting of slidewire 9|.

In both Figs. 1 and 'l, the primary measuring circuit and its relay 3i is used to adjust a slidewire (60, Fig. 1; I03, Fig. 7) in a second balanceable circuit which compensates in the circuit for the non-linearity of movement produced by the non-linearity of the condition-responsive devices or radiation pyrometers. It is to be understood the several features, particularly the system of Fig. 3, may be incorporated in the primary system or otherwise utilized to convert non-linear movement of the slidewire into linear changes of voltage in the measuring system or network proper. Conversely, linear adjustments or equal changes in the setting of slidewire m will produce nonlinear changes of voltage which. for certain aD- plications, will be found useful.

While preferred embodiments ofthe invention have been described, it will be understood that further modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims What is claimed is:

1. In combination, a condition-responsive means whose output varies non-linearly with respect to changes in the magnitude of a condition, a balanceable network including a variable impedance, circuit connections across said impedance for deriving from said network an output voltage determined jointly by the magnitude of said variable impedance and by a current whose magnitude is controlled by said impedance, a second network traversed by a constant current and including a variable impedance, and means for concurrently balancing the output of one network against the output of said condition-responsive means and for varying the impedance in the other of said networks whereby the output of said other network varies linearly with respect to changes in the magnitude of said condition.

2. In a temperature-difference control system, the combination of radiation pyrometers respectively responsive to difl'erent temperatures, each of said pyrometers having an output which varies non-linearly with temperature, a balanceable network, means for connecting first one and then the other of said pyrometers to said balanceable network, means for balancing said network first while one and then while the other of .said.

pyrometers is connected thereto, a second balanceable network including means for unbalancing it linearly with -respect to temperature in contrast with the non-linear unbalancing of said first network by said pyrometers, means operable concurrently with the balancing of said first network to operate said unbalancing means of said said second network, and a second means for balancing said second network.

3. In a temperature-diiference control system, the combination of radiation pyrometers respectively responsive to diiiering temperatures, each of said pyrometers having an output which varies non-linearly with temperature, a balanceable network, means for connecting first one and then the other of said pyrometers to said balanceable network, means for balancing said network first while one and then while the other of said pyrometers is connected thereto, a second balanceable network including means for unbalancing it linearly with respect to temperature in contrast with the non-linear unbalancing of said first network by said pyrometer, means operable concurrently with the balancing of said first network to operate said unbalancing means of said second network, a second means in said second network for balancing the same, means for introducing into said second network a voltage which in polarity and magnitude requires a predetermined temperature difference before unbalance of said second network may occur in one direction, and temperature-controlling means operable in response to unbalance of said second network in said one direction.

4. In a temperature-diiierence control system, the combination of radiation pyrometers each subjected to different temperatures, a measuring system, means for connecting said pyrometers in succession to said measuring system, said a linear unbalance, with respect to said temperatures, of said second network, means associated with said second ,network for in ing a voltage to establish a predetermined temperature diflerence before'unbalance thereof may occur in one directiomand means responsive to unbalance in said one direction for operatingsaid temperature-controlling means in the direction to limit said temperature diiference between said temperatures.

' 5. In a temperature-difierence control system, the combination of radiation pyrometers each subjected to difl'erent temperatures, a measuring system, means for connecting said pyrometers in succession to said measuring system, said meas-' uring system including a variable impedance for producing balance thereof, the movement of said variable impedance being non-linear with respect to changes of said temperatures, means for limiting the diilerence in magnitudes oi said temperatures, a second balanceabie network having a second impedance operable concurrently with said first-named impedance for producing unbal-' ancesaid network,meansforconnectingsaid pyrometers insuccession to'said primary network, a second balanceabie network including at least a second and a third adjustable impedance. means for adjusting one of said second and third impedances concurrently with adjustment of said impedance of said primary network to unbalance said second network, means operable when one oi said pyrometers is connected to said primary network for balancing said secondary network and inoperative to balance said secondary network when the other pyrometer is connected to said primary network, means operable ance in said second network, means for producing a linear unbalance, with respect to temperature, of said second network upon non-linear movement of said second impedance, means associated with said second network for introducing a voltage to establish a predetermined temonly when said other pyrometeris connected to said primary network for changing the balance of said secondary network by a predetermined amount and in predetermined sense, and means responsive to a resultant unbalance of said secondary network in a predetermined sense for controlling the operation of said temperature-changing means.

8."I'he combination with a regenerative furnace having a plurality of regenerative chambers respectively, containing heat absorbers and means for selectively controlling the flow of combustion materials through said chambers, of radiation pyrometers disposed in view of the interiors of said chambers and responsive'non-linearly to the temperatures developed therein, means for producing from said non-linear responses of said pyrometers atemperature-diiference response corrected for said non-linearity comprising a measuring circuit including a variable resistor, means for adjusting said variable resistor in resp nse to the diiference between said non-linear responses, a source of potential, a second resistor, means connecting said second resistor and said variable resistor inserles-circuit relation across said source, said variable resistor being adjustable to change the resistance of, and the current flowing in, said series circuit, an electrical circuit connected across said variable resistor for applying to said measuring circuit a voltage ance of, and the current flowing in; said series a power law, means responsive to unbalance of.

said primary network for adjusting said one of said adjustable resistors, and manual means for adjusting said other resistor to vary said root of said voltage, to produce any desired compensation for the power law voltage variationin said other network.

I. The combination with means for changing the relative magnitudes of temperatures, of a temperature-difference control system comprising a radiation pyrometer responsive to each of .said temperatures, each said pyrometer having an output which varies non-linearly with temperature, a primary measuring network including a variable impedance operable to a position indicative of the output of a pyrometer to balderived from said series circuit, said voltage having a magnitude dependent upon both the current change and the resistance change resulting from said adjustment of said resistor, and means responsive to a predetermined unbalance of said measuring circuit for controlling operation of said selective means.

9. The combination with a regenerative furnace having a plurality of regenerative chambers respectively containing heat absorbers and means for selectively controlling the flow of combustion materials through said chambers, of radiation pyrometers disposed in view of the interiors of said chambers and responsive to the temperatures developed therein in accordance with a power law thereof, means for producing from said power law responses of said pyrometers a temperature-difference response corrected for said power law responses comprising a measuring circuit including a variable resistor, means ior adjusting said variable resistor in response to the diiference between said power law responses, a sourceof potential, a second resistor, means connecting said second resistor and said variable resistor in seriescircuit relation across said source, said adjustable resistor being adjustable to change the resistance of, and the current flowing in, said series circuit, an electrical circuit connected across said variable resistor for applying to said measuring circuit a voltage derived from said adiustable resistor, said voltage having a magnitude dependent upon both the current change and the r sistance change resulting from said adjustment of said resistor, said resistors having values which for equal movements of said adjustable resistor vary said voltage substantially as a corresponding root of said power law changes thereby to correct for said power law responses oi said pyrometers, and means responsive to a predetermined unbalance of said measuring circuit for controlling operations oi said selective means.

10. Means for controlling temperature in response to the diii'erence between two temperatures by temperature-responsive devices whose outputs vary non-linearly with temperature, comprising a measuring network, movable means for balancing said network first in response to one of said devices and then in response to the other of said devices, a second network, means operable with said movable means for linearly unbalancing said second network in accordance with temperature, and control means operable from one controlling position to another in response to a predetermined unbalance in said second network corresponding to a predetermined diiference between said two temperatures.

11. The combination with a condition-responsive device having with respect to the condition under measurement a non-linear output characteristic, of a source 0! potential, at least a pair of resistors connected in series-circuit relation across said source, means operable in accordance with said non-linear output of said device for adjusting at least one of said resistors to change the resistance 01', and the current flowing in, said series-circuit for development across said adjustable resistor 01' a voltage whose magnitude varies non-linearly for equal changes of resistance values of said adjustable resistor and whose magnitude varies linearly with respect to changes of the condition under measurement, a circuit connection across said adjustable resistor for deriving from said series-circuit said voltage, and a balanceable network including said circuit connection for development in said network or said voltage which varies linearly with respect to change of the condition under measurement.

12. The combination with a condition-responsive device having with respect to the condition til under measurement a non-linear output characteristic, of a source or potential, a pair of resistors connected in series-circuit relation across said source, means operable in accordance with said non-linear output of said device for adjusting at least one of said resistors to change the resistance of, and the current flowing in, said seriescircuit for development across said adjustable resistor or a voltage which varies non-linearly with respect to equal changes in the resistance values of said adjustable resistor and linearly with respect to change of the condition under measurement, said resistors having values which for equal resistor adjustments vary said voltage substantially as a root or the magnitude of said resistor adjustments, a circuit connection across said adjustable resistor, and a balanceable network including said circuit connection for development in said network of said voltage which varies approximately linearly with respect to change of the condition under measurement.

ELWOOD T'. DAVIS.

REFERENCES crrEn The following references are of record in the 950,513 Northrup Mar. 1, 1910 1,162,475 Gibson Nov. 30, 1915 1,400,077 Keith Dec. 13, 1921 1,560,951 Thompson Nov. 10, 1925 1,582,060 Lovejoy Apr. 27, 1926 1,655,276 Lichtscheindl Jan. 3, 1928 1,677,691 Smith July 17, 1928 1,683,809 Schofield Sept. 11, 1928 1,760,204 Mittendori May 27, 1930 1,931,799 Hunter Oct. 24, 1933 2,098,574 Doyle Nov. 9, 1937 2,275,317 Ryder Mar. 3, 1942 2,329,841 Keinath Sept. 21, 1943 2,356,269 Potter Aug. 22, 1944 FOREIGN PATENTS Number Country Date 563,714 Germany Nov. 9, 1932 

